Cyclic secondary oil recovery process



May 6, 1969 w. B. FULTON ET AL 1 3,442,331

CYCLIC SECONDARY OIL RECOVERY PROCESS Original Filed May 27, 1964 PRODUCT/ON REsERvolR RATE /0 PREssuRE -P5/ FIG. 20 F|G.2b 60 FlG.2c 60 FlG. 2d60 i FIG.30 G0 FlGQ3b F|G.3c e0 Flasd WALTER B. FULTON LEONARD J.W/cKE/vHAL/sg INVENTO A TTORNE Y United States Patent 3,442,331 CYCLICSECONDARY OIL RECOVERY PROCESS Walter B. Fulton and Leonard J.Wickenhauser, Laurel, Miss., assignors to Central Oil Company, Laurel,Miss., a corporation of Mississippi Continuation of application Ser. No.370,526, May 27, 1964. ,This application May 22, 1967, Ser. No. 640,427

- Int. Cl. E21b 43/16, 43/20 US. Cl. 166-263 9 Claims ABSTRACT OF THEDISCLOSURE The above identified application is a continuation ofapplication No. 370,526, filed May 27, 1964, now abandoned. Thisinvention relates to the production of petroleum' by secondary recoverymethods, and in particular concerns an improved water flooding processwherein oil recovery is enhanced by maintenance of reservoir pressure.Among the various methods presently employed for recovering petroleumfrom oil bearing formations which have become depleted to the pointwhere petroleum no longer flows naturally into wells penetrating suchformations, the secondary recovery method commonly known as waterflooding probably enjoys widest exploitation. In brief, water floodingas usually practiced consists essentially in pumping water into one ormore injection wells under a pressure suflicient to cause the water toflow out through the oil bearing formation toward one or more otherwells which serve as producing wells. Theoretically, as the water flowsthrough the oil bearing formation the mobile oil contained therein isforced ahead of the advancing water front into the producing wells fromwhich it may be pumped to the earths surface. In many instances,however, this method proves ineffective by reason of'the occurrence ofstrata of high permeability extending-between the injection andproducing wells. The water. chooses the path of least resistance, i.e.,the strata of highest permeability, and hence travels from the injectionwells to the producing wells in more or less well defined channels andfails to sweep the oil from the formation as a whole.

Various means have heretofore vbeen suggested for preventing or stoppingsuch channeling of the water through the formation, as shown for examplein Patent No. 2,988,142 to Maly and in Patent No. 3,032,101 to Woertz etal. Such methods, however, usually contemplate some sort of plugging orpartial plugging of the more permeable strata of the formation so thatthe water is less likely to channel through such strata.

Many other methods have heretofore been proposed for increasing theproportion of oil in place which is recoverable from petroleumreservoirs by secondary recovery methods. Note for example the methodsdescribed in US. Patents 1,099,170, Dunn; 1,242,557, Dunn; 2,135,- 319,Bays; 2,724,438, Whorton et al.; 3,036,631, Holbrook; 3,084,743, West etal., and in the August 1963 issue of Journal of Petroleum Technology,pp. 877-884. None of the methods heretofore used, however, has beenfound to be consistently effective for the recovery of more than aboutfifty to sixty percent of the mobile oil in place.

-It is an object of this invention to provide a secondary 3,442,331Patented May 6, 1969 recovery process which is effective for therecovery of extremely high proportions of the oil in place in areservolr.

Another object of this invention is to provide a secondary recoveryprocess wherein channeling of water or other liquid through morepermeable strata of the formation is substantially reduced.

Still another object of this invention is to provide a water injectionprocess wherein both the more permeable and the less permeable porespaces of the rock are pressurized under stabilized conditions so as tocause production of oil therefrom.

Yet another object of the invention is to provide a process wherebygravity segregation of the gas, oil and water in the formation isimproved by liquid injection.

Another object of the invention is to provide a process wherein watermay be injected into a series of interbedded lenticular sands within asingle well bore even though each sand may have different rockcharacteristics.

A further and most important object of the invention is to provide asecondary recovery process wherein maximum recovery of oil from aformation may be obtained without having to pump the production wells atany stage of the production.

These and other objects of the invention are attained by a waterinjection process wherein production from a partially depleted reservoiris substantially completely shut off before any injection is begun. Thepressures at the bottoms of all of the wells in the formation are thenmonitored until they are stabilized. A pressurizing fluid, usuallywater, is then injected into one or more injection wells until thepressure reaches a desired point which preferably is approximately theoriginal virgin reservoir pressure. When the desired pressure has beenreached injection is stopped and pressures at bottoms of all wells inthe reservoir are again monitored until the pressures are stabilized.Then, without any further injection, the producing wells are opened andallowed to flow. Production is continued until flowing production ratesare reduced to a value where economics dictate that further pressurizingof the reservoir would be advantageous, at which time the productionwells are again shut in and the injection cycle' is repeated.

For a better understanding of the invention reference is now made to theaccompanying drawings wherein FIGURE 1 is a graph showing typicalvariation in reservoir pressure and production rates during practice ofone embodiment of the present invention;

FIGURES 2(a) through 2(d) are schematic representations of theprogression of a direct line drive fluid injection system as taken fromFIGURE 7.26 of Applied Petroleum Reservoir Engineering, by Kraft andHawkins (Prentiss Hall, 1959); and

FIGURES 3(a) through 3 (d) are schematic representations similar toFIGURES 2(a) through 2(d) showing the progression of a water injectionsystem carried out according to the present invention.

The process of this invention may be readily understood by reference toFIGURE 1, wherein the point 10 signifies the initial virgin pressure ofa reservoir before any production therefrom. As oil is produced from thereservoir, the reservoir pressure is reduced as indicated by the line12. At the same time the rate of production in barrels per day isreduced as indicated by line 16 from a maximum at point 14. Thus after aperiod of time, as reservoir pressure is depleted the amount of oilwhich flows from the well decreases until the rate of production is'very low, as indicated at point 18, or flowing production ceasesentirely. At this point the pressure in the well is as indicated atpoint 20 in the upper curve in FIGURE 1. Preferably in the performanceof this process this pressure is still above the bubble point of thereservoir. In other words it is preferred that the initial productionfrom the reservoir be stopped before the reservoir pressure is reducedbelow that required to maintain the reservoir in an undersaturatedcondition.

In some instances the production well or wells may be put on pump duringthis initial production period, but in most cases it will be mostdesirable to cease the initial production prior to the time that it isnecessary to pump.

Following this first cessation of production all outlets from thereservoir are preferably shut in for a period of time suflicient toallow reservoir pressures to become stabilized. It will be understoodthat during production from a well the reservoir pressure immediatelyadjacent the production well will be substantially less than thepressure within the reservoir some distance away from the productionwell. However, when production ceases and the production well is shutin, any disparity in pressures at the same level within the reservoir issubstantially eliminated, since the reservoir acts as any other tankcontaining a fluid under pressure. Due to the small passageways in thesand of the reservoir it may in some cases take a considerable time, upto several days, before the pressures within the reservoir becomestabilized. During this time the pressure at the bottom of a productionwell will gradually increase, and the pressure elsewhere in thereservoir will gradually decrease. Stabilization can be determined bypressure measuring instruments at the bottom of various wells whichencounter the reservoir. When these pressure measuring instrumentsindicate that pressure changes in the reservoir have ceased, then it maybe assumed that pressures within the reservoir have stabilized.

Following the stabilization of pressures in the reservoir, whichstabilization period is indicated on the pressure curve in FIGURE 1 at22, water injection into the formation is begun. This initial injectionperiod is indicated in FIGURE 1 by the curve 24, showing increase inreservoir pressure, and by the period between point 18 and point 26 onthe production curve wherein there is no production from the well. Theusual water flooding equipment may be utilized to inject water into theformation as rapidly as possible or as desired. In many cases injectionpressures at the injection well will be low, since the weight of thecolumn of water in the injection well will be sufiicient to cause theinjection water to flow into the reservoir at a high rate. Where highinjection rates are required, and where formations otfer resistance tothe injection of water at desired rates, it will be necessary to injectwater by means of conventional water injection pumps. Injectionpressures should normally be maintained at a level below which rupturingof the formation can occur.

As indicated on the pressure curve in FIGURE 1, when sufiicient liquidhas been injected to bring the reservoir pressure up to approximatelythe original virgin pressure of the reservoir, injection is stopped, asindicated by the point 28. Pressures somewhat higher than virginpressure may be used, but pressures should normally be maintained at alevel below which rupturing of the formations can occur.

After the desired pressure has been reached preferably, a period of timeis provided during which there is no injection and no production fromthe reservoir, and during this period pressures within the reservoir areallowed to stabilize until pressure changes cease. Such stabilizationperiod is indicated by the time interval 30 marked on the pressure curvein FIGURE 1.

As soon as stabilization of the reservoir pressures has been reached,the production wells may again be opened and production continued untilsuch time as the reservoir pressure has been reached a substantialamount to whatever level is considered by the operator to be a propertime for ceasing production and again initiating injection to increasethe reservoir pressure. As shown in FIGURE 1, production periods afterthe initial production period may be substantially shorter and may be athigh average production rates. Thus, as shown in this figure, the lines32, 34 and 36 on the pressure curve indicate production periods whichcorrespond to the periods of lines 38, 40 and 42 on the production ratecurve, and lines 44 and 46 indicate intermediate injection periodsduring which there is no production. The small steady pressure periods48, 50, 52, 54 and 56 on the pressure curve indicate the pressurestabilization periods provided before and after each injection period.

In a conventional water injection operation, injection and productiontake place at the same time. Thus there is created within the reservoiran area of lowest pressure, i.e., at the production well; and an area ofhighest pressure, i.e., at the injection well. Thus, as in any hydraulicsystem, there is a tendency for the injection water to flow from thehigh pressure point to the low pressure point. Such flow will naturallyfollow the path of least resistance, which in a reservoir is the strataof the highest permeability with flow along the shortest paths to theproducing outlets. Thus conventional fluid injection will displace oilprimarily from the more permeable strata of sand, leaving substantialquantities of oil in less permeable strata.

Thus in the usual fluid injection system the path taken by the injectionfluid in substantially homogeneous sand reservoirs is most likelyapproximately that shown in the four drawings, FIGURE 2(a) through 2(d),wherein 60 represents the injection well, 62 represents a productionwell, and the hatched area 64 represents the advancing fluid which isinjected through the injection well While oil is being produced from theproduction well. As seen in FIGURE 2(a), the fluid may initially startout with a substantially cylindrical front, but tends to flow mostreadily toward the lower pressure area at production well 62, so that,as shown in FIGURES 2(b) and 2(0), the injection fluid front is movedmuch more rapidly in the direction of the production well than in otherdirections from the injection well. Thus eventually, as shown in FIGURE2(d), the injection fluid is flowing directly to the production wellwithout sweeping out any of the area on either side of its path of flow.

The advance of the injection water through the formation when injectedaccording to the process of this invention is illustrated in FIGURES3(a) through 3(d). When the process of this invention is practiced,there is no point of lower pressure at the production well, or elsewherein the formation, during the time water is being injected. Thus there isno tendency for the injection water to flow in any particular horizontaldirection so that it will flow substantially uniformly in all directionsfrom the injection well. It is therefore thought that a substantiallycylindrical front will be obtained which will gradually, as shown inFIGURES 3(a), 3(b) and 3(a), move in all directions from the injectionwell to sweep a high proportion of the reservoir as the water movesoutwardly from the injection well. It will be apparent from a comparisonof FIGURES 2(d) and 3(d) that the area swept by the injection water inthe process of this invention is substantially greater than that in thearea swept by the prior art processes.

The stabilization of pressure gradients provides further benefit in thatgravity segregation of fluids in the reservoir is facilitated. Sincepressure gradients are at a minimum, the tendency is for water, becauseof its greater density, to move downwardly by gravity through thereservoir rock and seek a level below the oil in the reservoir.

In the practice of this process to date the production wells have beencompletely shut down during water injection periods. However, theadvantageous results of this invention may be obtained by producing atvery low rates during the injection period, if the production rates arekept low enough that the pressure reduction in the reservoir resultingtherefrom is more or less localized around the production well, beinglow enough that no noticeable pressure gradient extends to the area intowhich water is being injected.

It is apparent that the process of this invention may be used as it hasin the past with a single injection well and a single production wellintersecting the reservoir, and that it also may be used where there areany number of production wells together with one or more injectionwells,as required for an eflicient flooding operation. It is of courseessential for the pressure stabilization steps that all wellsencountering the reservoir be controlled and made a part of the process.

Preferably the injection wells used for the practice of this inventionwill be spaced farther away from the production wells than in theconventional water flood. There is no need for them to be close to theproduction wells, since the entire reservoir is pressurized. If theinjection wells are farther away, the period of time required for theinjection water to reach the production well is delayed. One or morewater input wells are preferably located along the flanks of thereservoir to allow more beneficial effects from gravity segregation.However, in other instances, since water does nothave the tendency toflow through oil saturated rocks, injection wells may be located inother portions of the rservoir without appreciable damage due tochanneling;

The process of this invention may most advantageously be practiced byrepressuring the reservoir after each production period to a pressuresubstantially equal to virgin reservoir pressure. However it may in someinstances be found advisable to raise the pressure slightly higher thaninitial reservoir pressure to some point less than formation breakdownpressure. Also in other instances it may be found most advisable to stopinjection before the initial reservoir pressure has been reached andstart produc tion then at a lower pressure. Such procedures will producesatisfactory results so long as the process of this invention isfollowed wherein stabilization of pressures within the reservoir isallowed both before any injection is begun and after all injection hasbeen completed.

Since no production takes place while injection is going on, it is mostadvantageous economically to inject as rapidly as possible. This hasbeen found to be no problem, however, since injection rates may normallybe six to nine times as high as rates of production, so that a period ofone month provided for injection will raise the reservoir pressure highenough to allow six to nine months of flowing production. Oil productionrates after an injection by the process of this invention are oftensubstantially as high as original production rates from the reservoirand therefore substantial increases in annual production are stillobtained by the process of this invention even though the reservoir isshut in for one or two months per year.

It will be apparent that the process of this invention may be carriedout in a plurality of sands encountered by a well merely by pressurizingeach sand to a desired pressure with suitable plugs between sands.

The process of this invention greatly increases the ability to produceoil without high proportions of water, because under the pressurestabilized conditions that exist during production according to thisinvention, oil will flow through the oil-saturated rock, in preferenceto water, toward the oil-producing outlets.

One of the major advantages obtained by the process of this invention isthe elimination of any necessity to pump in most reservoirs, since thereservoir pressure is most advantageously maintained high enough at alltimes to cause the wells to flow. Thus the need for expensive artificiallifting equipment and consequent high operating cost is greatly reducedif not entirely eliminated. Other savings may be realized because of thefact that injection pumps and facilities are required only occasionally,so that the same equipment may be moved from reservoir to reservoir asinjection is required.

The process of this invention was performed in a closed reservoir havingan areal extent of approximately 200 acres. A single production well wascompleted with perforations from 11,661 to 11,681 feet. Original bottomhole pressure was 5750 p.s.i. and the bubble point was 1262 p.s.i. Thewell originally flowed, on inch choke, 422 barrels per day with aflowing surface pressure of 1350 p.s.i. After flowing 90,000 barrels ofoil, the well died and was placed on hydraulic pump. After some threeyears on pump, the well was still capable of pumping 50 barrels of oilper day. However, the well was closed in at a bottom hole pressure of1320 p.s.i. after cumulative production of 230,000 barrels of oil, and atotal reservoir withdrawal of 270,000 barrels.

Prior to closing in the production well, pipe had been set in anotherwell 1200 feet west. When the production well was closed in thepressures in both wells were monitored until reservoir pressure wasstabilized, and then water injection was started through the second welland continued until 200,000 barrels of water had been injected andbottom hole pressure was raised to 5300 p.s.i. Injection was terminatedand the bottom hole pressures were allowed to stabilize again. Bottomhole pressure surveys indicated slightly less than six days wererequired for stabilization. The production well was then opened andflowed on inch choke at the rate of 230 barrels per day with 5S0# tubingpressure. After approximately 50,000 barrels of reservoir fluids wereproduced, yielding 39,000 barrels of oil, the production well was againclosed in. Bottom hole pressure stabilized at 4050 p.s.i. Waterinjection was then again initiated at a rate of 1500 barrels per daywith a surface injection pressure of 700 p.s.i.

It will be noted that in this particular case the well had been pumpeduntil it was only marginal economically and probably would have had tobe abandoned in a relatively short time. However, by means of theprocess of this invention additional recovery amounting to nearly twentypercent of the primary production has already been ob tained and it isanticipated that upon continuation of utilization of the process of thisinvention the total recovery from this reservoir, without any additionalpumping of the well, will far exceed the primary recovery, and shouldamount to -90% of the total mobile oil in the reservoir.

Although specific embodiments of this invention have been shown anddescribed herein the invention is not limited to these embodiments butonly as set forth by the following claims.

We claim:

1. A secondary recovery process comprising:

substantially shutting off all outlets from a partially depletedreservoir, allowing pressure come stabilized, injecting liquid into saidreservoir sufficient to restore reservoir pressure to near virginconditions,

allowing pressure gradients within the reservoir to become stabilized,and

producing from said reservoir while substantially no liquid is beinginjected.

2. A petroleum production process comprising:

producing from a reservoir until the reservoir pressure has reducedsubstantially "below virgin conditions, substantially shutting 01f alloutlets from said reservoir, allowing pressure gradients within thereservoir to become stabilized,

injecting liquid into said reservoir sufficient to restore reservoirpressure to near virgin conditions,

allowing pressure gradients within the reservoir to become stabilized,and

producing from said reservoir while substantially no fluid is beinginjected.

3. A petroleum production 2 wherein initial production ment of a mobilegas phase.

4. A petroleum production process comprising:

producing oil from a reservoir until the reservoir gradients within thereservoir to be process as defined by claim is stopped prior todeveloppressure has been reduced substantially below virigin conditions,

reducing the production rate to such a level that pressure gradientswithin the reservoir may become substantially stabilized, maintainingsaid reduced production rate after said substantially stabilizedcondition is reached,

beginning and continuing water injection into said reservoir, withoutany increase in production rate, until the reservoir pressure has beenincreased substantially but to a pressure below breakdown pressure,

substantially terminating said injection,

allowing pressure gradients within the reservoir to again becomestabilized, and

resuming oil production at increased rates while substantially no liquidis being injected.

5. A secondary recovery process comprising:

substantially shutting ofl? all outlets from a partially depletedreservoir,

allowing pressure gradients within the reservoir to become stabilized,injecting water into said reservoir at approximately the maximumfeasible rate until the reservoir pressure is restored to approximatelyvigin conditions,

allowing pressure gradients within the reservoir to become stabilized,and

producing oil from said reservoir without further injection.

6. A secondary production process for use with a reservoir having aproduction well and an injection well comprising:

shutting down said production well,

allowing pressure gradients within the reservoir to become stabilized,

repressuring said reservoir by injecting water through said injectionwell while said production well is shut down,

stopping water injection,

allowing pressure gradients within the reservoir to become stabilized,and

resuming production from said production well while no water is beinginjected,

7. A secondary recovery process wherein the steps set forth in claim 6are repeated until the reservoir is substantially depleted ofrecoverable petroleum.

8. A secondary recovery process wherein the steps set forth in claim 6are performed before the reservoir pressure has ever been reduced enoughto cause an under saturated condition.

9. A secondary recovery process as defined by claim 6 wherein the saidwater injection is carried out at substantially the maximum rate atwhich the formation can take water.

References Cited UNITED STATES PATENTS 5/1966 Cooke et al 166-9 OTHERREFERENCES STEPHEN J. NOVOSAD, Primary Examiner.

US. Cl. X.R. l66-268

